U.S. patent number 6,422,059 [Application Number 09/630,112] was granted by the patent office on 2002-07-23 for apparatus for detecting changes in concentrations of components of fluid mixtures.
This patent grant is currently assigned to Calgon Carbon Corporation. Invention is credited to Mark Allen Bollinger, David T. Doughty, Micahel Greenbank.
United States Patent |
6,422,059 |
Greenbank , et al. |
July 23, 2002 |
**Please see images for:
( Certificate of Correction ) ** |
Apparatus for detecting changes in concentrations of components of
fluid mixtures
Abstract
An apparatus is provided for the detection of changes in
concentrations or amounts of components of a fluid mixture by
measuring changes in an electrical or a mechanical property of a
sensing element when the sensing element is exposed to the fluid
mixture. The preferred property of the sensing element to be
measured is its electrical resistance which changes when the
sensing element adsorbs at least one component of the fluid
mixture.
Inventors: |
Greenbank; Micahel (Monaca,
PA), Doughty; David T. (Moon Township, PA), Bollinger;
Mark Allen (Pittsburgh, PA) |
Assignee: |
Calgon Carbon Corporation
(Pittsburgh, PA)
|
Family
ID: |
24525823 |
Appl.
No.: |
09/630,112 |
Filed: |
August 1, 2000 |
Current U.S.
Class: |
73/23.2; 204/294;
422/70 |
Current CPC
Class: |
G01N
27/12 (20130101) |
Current International
Class: |
G01N
27/12 (20060101); G01N 019/10 (); G01N 030/02 ();
C25B 011/12 () |
Field of
Search: |
;73/23.2,23.28,23.4
;204/294,403 ;205/560,743 ;422/70,98 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Williams; Hezron
Assistant Examiner: Politzer; Jay L.
Attorney, Agent or Firm: Cohen & Grigsby, P.C.
Claims
What is claimed is:
1. An apparatus for detecting changes in concentrations of minor
components of a fluid mixture comprising two sensing elements in a
housing, through which the fluid mixture is conducted, said sensing
elements comprising activated carbon cloth and exhibiting a
measurable electrical resistance in response to any said
concentration changes when said sensing elements are exposed to or
in contact with said fluid mature, and a means for detecting said
electrical resistance.
2. An apparatus for detecting changes in concentrations of minor
components of a fluid mixture as recited in claim 1 further
including an adsorbent disposed between said sensing elements.
3. An apparatus for detecting changes in concentrations of minor
components of a fluid mixture as recited in claim 1 wherein said
activated carbon cloth is capable of adsorbing at least one
component of said fluid mixture.
4. An apparatus for detecting changes in concentrations of minor
components of a fluid mixture as recited in claim 1 wherein said
sensing elements have substantially equal electrical resistance
before exposure to or contact with said fluid mixture.
5. A method for detecting changes in concentrations of minor
components of a fluid mixture comprising the steps of: a.
conducting a stream of the fluid mixture sequentially through
thesensing elements of the apparatus of claim 1 or 2; b. measuring
an electrical resistance of each of the sensing elements; c.
recording a change in the electrical resistance of the first
sensing element relative to the electrical resistance of the second
sensing element; and d. associating the change in the electrical
resistance of the first sensing element with the change in the
concentrations of components of the fluid mixtures.
6. A method for detecting changes in concentrations of minor
components of a fluid mixture comprising the steps of: a.
conducting a stream of the fluid mixture sequentially through the
sensing elements of the apparatus of claim 3, b. measuring an
electrical resistance of each of the sensing elements; c. recording
a change in the electrical resistance of the first sensing element
relative to the electrical resistance of the second sensing
element; and d. associating the change in the electrical resistance
of the first sensing element with the change in the concentrations
of components of the fluid mixture.
7. A method for detecting changes in concentrations of minor
components of a fluid mixture comprising the steps of: conducting a
stream of the fluid mixture sequentially though thesensing elements
of the apparatus of claim 4, b. measuring an electrical resistance
of each of the sensing elements; c. recording a change in the
electrical resistance of the first sensing element relative to the
electrical resistance of the second sensing element; and d.
associating the change in the electrical resistance of the first
sensing element with the change in the concentrations of components
of the fluid mixture.
8. An apparatus for detecting changes in concentrations of minor
components of a fluid mixture as recited in claim 1, wherein said
housing comprises a first chamber and a second chamber, one of said
sensing elements being disposed in said first chamber and the other
of said sensing elements being disposed in said second chamber.
9. An apparatus for detecting changes in concentrations of minor
components of a fluid mixture as recited in claim 8, further
including an adsorbent disposed between said sensing elements.
10. An apparatus for detecting changes in concentrations of minor
components of a fluid mixture as rented in claim 1 or 8 wherein
said cloth comprises of a woven or nonwoven fiber selected from the
group consisting of natural fiber, man-made fiber, synthetic fiber
and a combination thereof.
11. An apparatus for detecting changes in concentrations of minor
components of a fluid mixture as recited in claim 1 or 8 wherein
said cloth comprises an electrically conducting sheet having
activated carbon particles therein.
Description
FIELD OF THE INVENTION
The present invention relates to an apparatus for detecting changes
in concentrations of components of a fluid mixture by relating
changes in an electrical or mechanical property of a sensing means
to such changes in concentrations. In particular, this invention
relates to such an apparatus based on a change in a resistance of a
sensing means made of an activated carbon material as the
concentrations change.
BACKGROUND OF THE INVENTION
Due to the capability of activated carbon to remove a wide variety
of chemical compounds from fluid mixtures, it has been widely used
to purify such mixtures. The removal is effected by physical
adsorption of these contaminants in the micropores of the activated
carbon which are developed during the manufacturing process. The
activated carbon is said to be spent when its micropores are filled
with the contaminants and it no longer has any capacity for further
adsorption. When the activated carbon bed is spent, the
concentration of a contaminant in the treated fluid stream
increases rapidly. This event is referred to as the breakthrough of
the go contaminant. To ensure a properly treated stream, the
activated carbon bed must be replaced at some time prior to the
first breakthrough of the contaminants. Since the concentrations
and the types of contaminants may vary during the time during which
the carbon bed is on stream, the breakthrough point cannot be
precisely predicted and, therefore, the quality of the treated
fluid stream must be monitored by frequent periodic testing, which
sometimes can be costly. Therefore, there is a need for a device
that can warn the user when the carbon bed is nearly spent without
the requirement for periodic testing of the treated fluid. Such a
device preferably stays on stream continuously and detects an
increase in the contaminant level near the outlet of the carbon bed
to provide an efficient use thereof. Such a device is also useful
in purification systems employing media other than activated
carbon.
Many sensors have been proposed in the art, but they have been
directed to detecting only hydrocarbon contaminants and, thus,
limited in their applications. U.S. Pat. Nos. 5,079,944 and
5,150,603 disclosed hydrocarbon vapor sensors and systems for
detecting leaks from underground storage tanks. The heart of the
sensors of these patents was a sensing element made of a conductive
polymer, the resistance of which changed when hydrocarbons absorbed
into and swelled the polymer matrix. In one embodiment, the
conductive polymer comprised a polymeric tape carrying conductive
carbon particles. In another embodiment, the conductive polymer was
an elastomer or a silicone rubber filled with silver-coated glass
spheres or metallic silver flakes. The presence of hydrocarbon
vapor was ascertained when the resistance of the sensing element
increased rapidly. However, this sensor would not be able to detect
any contaminant that does not absorb readily into and swell the
polymer matrix.
Moyer et al. disclosed the use of an end-of-service-life indicator
for organic vapor cartridge respirators (E. S. Moyer et al., "A
Preliminary Evaluation of an Active End-of-Service-Life Indicator
for Organic Vapor Cartridge Respirators," Am. Ind. Hyg. Asssoc. J.,
Vol. 54, No. 8, pp. 417-425 (1993)). The sensor of this study
comprised a mixture of silicone rubber and carbon particles
deposited on a substrate. The detection of organic vapor also
relied on the swelling of the silicone matrix, resulting in an
increase in the resistance of the silicone/carbon conducting film.
Thus, a contaminant that does not swell the silicone would not be
detected because the resistance of the film would not change.
Marchand reported the construction of an organic vapor sensor which
comprised a small bed of activated carbon cloth (E. G. Marchand,
Ph.D. Thesis, Michigan Technological University, 1996). Experiments
under flow conditions were done only with trichloroethylene in dry
air. The electrical resistance of the carbon cloth began to
increase sharply when the challenge gas is admitted into the carbon
cloth sensor long before the breakthrough occurred. There was no
suggestion as to how a sensor may be configured to indicate
reliably a breakthrough by directly relating a change in this
electrical property to the breakthrough point.
Therefore, it is an object of the present invention to provide an
apparatus for detecting changes in concentrations of a range of
minor components of a fluid mixture. It is a further object of the
present invention to provide a detection apparatus that relates the
change in electrical resistance, or mechanical properties, of a
carbon-based sensing element to the change in concentrations of the
minor components. It is still another object of the present
invention to provide an apparatus for detecting the breakthrough of
minor components of a fluid mixture from an activated carbon bed
used for the purification thereof.
SUMMARY OF THE INVENTION
The present invention provides an apparatus for detecting changes
in concentrations, or amounts of minor components, of a fluid
mixture that measures the changes in electrical resistance or
mechanical properties of a sensing element when the sensing element
is exposed to the fluid mixture. Minor components of a fluid
mixture, as are referred to herein, are those having weight-based
or molar concentrations less than about 50 percent. Such an
apparatus will hereinafter also be referred to as a detector. The
present invention measures changes in other electrical or
mechanical properties of the sensing element, such as electrical
capacitance, mechanical strength or dimensional change, when such
changes occur in relation to changes in concentrations or amounts
of the minor components of the fluid mixture. When the measured
electrical or mechanical property of the sensing element changes,
the detector indicates a change in a concentration. The means for
detecting the measurable property varies depending upon the
property and includes, for example, electrical detectors,
Wheatstone bridge related technologies or stress or strain gauges.
The present invention further includes, in another embodiment, a
sensing element for use in detecting changes in concentrations
minor components of a fluid mixture, said element comprising an
activated carbon cloth having a connecting means for measuring said
changes.
In general, the detector of the present invention comprises at
least first and second sensing elements that are carbon-based, or
are of other suitable materials, such as metal, and have
substantially the same composition, electrical resistance and
dimensions. The electrical resistances of the two sensing elements
preferably differ by less than 10 percent; more preferably, less
than 5 percent. One particularly suitable carbon for the
manufacture of the sensing elements is activated carbon fiber or
activated carbon cloth ("ACC") which is manufactured in thermal
processes from raw materials made of woven or non-woven natural,
man-made, or synthetic fibers and which possesses a high surface
area for adsorption of a wide variety of compounds. ACC typically
has a low electrical resistance, and this resistance changes
substantially when the ACC adsorbs even a small amount of
contaminants. This property is used advantageously in the detector
of the present invention to detect contaminants in fluid
streams.
Alternatively, the sensing elements may be made of an electrically
conducting sheet comprising a conductively effective amount of
activated carbon particles immobilized in a polymeric fiber
mixture. The present invention may be used in conjunction with a
system, such as a purification system. When used with a
purification system, the first sensing element is exposed to the
fluid mixture, the change in the concentration of one or more minor
components is detected. Preferably, the first sensing element is
positioned at a location where the fluid mixture initially has a
first substantially stable composition. The second sensing element
serves as a reference and remains exposed to the same fluid mixture
having the first substantially stable composition. In many
situations in which the fluid mixture is being purified by the
removal of the minor components, the fluid mixture having the first
substantially stable composition is that substantially devoid of
the minor components. As the concentration of a minor component in
this mixture increases, the changes in such concentration are
detected. Initially, the electrical resistances or mechanical
properties of the first and second sensing elements are
substantially equal when exposed to the same fluid mixture having
the first substantially stable composition. When the concentration
of a minor component of the fluid mixture begins to change,
measurable electrical or mechanical properties of the first sensing
element also begin to change in comparison to that of the second
sensing element when exposed to the fluid mixture near the outlet
of the purification system. The present invention will detect that
a minor component of the fluid mixture is about to breakthrough and
a warning signal may be given.
In a preferred embodiment of the present invention, the detector
comprises two sensing elements made of ACC which are spaced apart
in an elongated enclosure having an inlet and an outlet. An
adsorbent capable of removing the minor components of the fluid
mixture is disposed in the elongated enclosure between the sensing
elements. Examples of such an adsorbent are activated carbon that
have the form, for example, of granules, pellets, or spheres;
natural or synthetic zeolite; silica gel; activated alumina; and
any of these adsorbents modified to enhance their adsorption
capacities. Electrically conducting leads are provided to measure
the electrical resistance of each of the carbon-cloth sensing
elements. The fluid mixture, having a first substantially stable
composition, enters the detector and contacts, in sequence, the
first sensing element, the adsorbent portion, and the second
sensing element. This fluid stream is preferably a split stream of
the fluid mixture flowing through the enclosure and is taken from a
point near the outlet of the purification system. Initially, the
compositions of the fluid contacting the first and second sensing
elements are substantially the same and both sensing elements
exhibit substantially the same electrical or mechanical properties.
The properties are used to establish a base line.
When used with a purification system and the capacity of the
purification system has been exhausted, the concentrations of the
minor components of the fluid mixture begin to increase. The
adsorption of the minor components in the ACC material of the first
sensing element results in a change in the electrical resistance
thereof. The change is positive or negative depending on the types
of the minor components and the fluid mixture. Due to the
adsorption of the minor components in the adsorbent portion between
the first and second sensing elements, the resistance of the second
sensing element remains at the base line value for a short time
thereafter. Thus, a differential in the resistances indicates that
a breakthrough of the minor components from the purification system
is about to occur. A means for detecting the differential is
provided and can be designed to notify an operator of the system so
that appropriate actions may be taken to ensure a continued quality
of the treated fluid stream. Several similar detectors of the
present invention may be used in conjunction with a purification
system to indicate the degree of exhaustion of the capacity of the
purification system. When used in this manner, the detectors are
arranged at intervals along the system.
In another embodiment of the present invention, each carbon-based
sensing element is housed in a separate enclosure. The first
sensing element is exposed to a stream of the fluid mixture, the
change in the composition of which is desired to be detected. This
first sensing element is located before the outlet of the
purification system to detect the minor components before they
breakthrough. The second sensing element serving as the reference
is always exposed to a split stream of the treated fluid. At the
beginning of the purification operation, the compositions of the
fluid streams flowing through the sensing elements are
substantially the same. Thus, the differential in the resistances
of the sensing elements is near zero. As the first minor component
of the fluid mixture reaches the location of the first sensing
element, its resistance begins to change rapidly while that of the
reference sensing element remains substantially constant. This
change signals that the first minor component is about to
breakthrough from the fluid treatment system.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a preferred embodiment of the
detector of the present invention.
FIG. 2 shows the response of an ACC sensing element of the present
invention with respect to changes in the concentration of carbon
tetrachloride in air.
FIG. 3 shows the response of an ACC sensing element of the present
invention with respect to changes in the concentration of SO.sub.2
in air.
FIG. 4 shows the response of an ACC sensing element of the present
invention with respect to changes in the concentration of ammonia
in air.
FIG. 5 shows the response of an ACC sensing element of the present
invention with respect to changes in the concentration of sucrose
in water.
FIG. 6 shows the response of an ACC sensing element of the present
invention with respect to changes in the concentration of isopropyl
alcohol ("IPA") in water.
FIG. 7 shows a schematic diagram of the second embodiment of the
present invention.
FIG. 8 shows the response of an ACC sensing element of the present
invention with respect to changes in the concentration of n-butane
in air.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A preferred embodiment of the present invention is substantially as
shown in the schematic diagram of FIG. 1. Detector 10 comprises a
housing section 20 having two open ends, a length of about 3 feet,
and a diameter of about 3 inches. Section 20 is preferably a
plastic pipe or, when the fluid mixture is under a superatmospheric
pressure, section 20 may be advantageously made of a metal. The
length and diameter of section 20 may be varied depending on the
application. For example, a length of less than 1 foot is generally
adequate for most applications. The diameter of section 20, as will
be seen, will determine the accuracy of the measurement of the
resistance of the sensing elements. A larger diameter is generally
preferred. Section 20 is sealed with end caps 22 and 24. End caps
22 and 24 include pipe fittings 32 and 34 for conducting a stream
of fluid into and out of section 20.
Sensing elements 40 and 50 are positioned in end caps 22 and 24.
When the end caps 22 and 24 and the section 20 are made of a metal,
each sensing element 40 or 50 is disposed between two
non-conducting gaskets (not shown) to electrically isolate it from
the detector housing. Each sensing element is made of one or more
layers of ACC preferably having a dimension such that it completely
covers the cross sectional area of the pipe section. Both sensing
elements are preferably made of substantially the same ACC material
so that their electrical resistances are substantially equal. Each
sensing element is provided with a zinc electrode 42 and 52
attached to the ACC material for a connection with electrical leads
60 and 62, respectively. Plastic screens 70 and 72 provide
mechanical support to the sensing elements 40 and 50.
In a preferred embodiment, an amount of an adsorbent 75 having a
large pore volume for adsorption of the first minor component of
the fluid mixture is contained in the pipe section 20 to provide a
substantial time between the moments when the first and second
sensing elements begin to contact the first minor component. This
adsorbent may be activated carbon, zeolite, activated alumina,
silica gel, or mixtures thereof.
Electrical leads 60 and 62 are preferably connected to a Wheatstone
bridge for the detection of changes in the electrical resistance of
the sensing elements. Initially, a stream of the fluid mixture
devoid of minor components is conducted through the present
invention and the Wheatstone bridge is balanced. When the
concentration of the first minor component of the fluid mixture
flowing through the detector begins to increase, the carbon cloth
of the first sensing element begins to adsorb this component,
resulting in a change in its resistance and an off-set voltage in
the Wheatstone bridge. This voltage may be used to warn the user of
the fluid purification system that the first minor component of the
fluid mixture has broken through at the point of the detection.
EXAMPLE 1
Detection of Carbon Tetrachloride
The detector of the present invention as is shown in FIG. 1 was
used to demonstrate the detection of carbon tetrachloride in air.
Each of the sensing elements was made of one layer of FM5/250 ACC
(Calgon Carbon Corporation, Pittsburgh, Pa.). A stream of air
having a flow rate of about 16 liter per minute (1/min.) and a
relative humidity (RH) of about 20% was conducted overnight through
the detector. The Wheatstone bridge was balanced and carbon
tetrachloride was introduced into the stream of air to obtain a
concentration of about 1000 ppm (by volume) CCl.sub.4. The
Wheatstone bridge immediately showed a large off-set voltage, as is
shown in FIG. 2, indicating a measurable change in the electrical
resistance of the first sensing element. The resistance of the
first sensing element immediately decreased from 31.1 ohm to 27.8
ohm while that of the second sensing element decreased from 31.7
ohm to 29.7 ohm. If this stream of air had been a split stream from
an air purification system which was designed to remove CCl.sub.4,
such a detection of the off-set voltage could be used to indicate a
breakthrough of CCl.sub.4 at the point of gas sampling. The flow
was continued until the adsorbent in the pipe section was saturated
with CCl.sub.4 and CCl.sub.4 was detected at the exit end of the
detector, at which time the off-set voltage began to decrease as
the difference between the resistances of the two sensing elements
began to decrease.
EXAMPLE 2
Detection of Sulfur Dioxide
A detector having the same construction and dimension as that used
in Example 1 was tested for the detection of SO.sub.2 in a stream
of air. Air at a flow rate of about 32 l/rain. and about 50% RH was
conducted through the detector overnight through the detector. Then
the Wheatstone bridge was balanced and SO.sub.2 was introduced into
the air stream to obtain a concentration of about 550 ppm by
volume. The Wheatstone bridge immediately showed a large positive
off-set voltage, as is shown in FIG. 3, indicating a large change
in the electrical resistance of the first sensing element. The
resistance of the first sensing element immediately decreased from
25.7 ohm to 6.9 ohm while that of the second (reference) element
decreased from 13.3 ohm to 10.7 ohm. If this stream of air had been
a split stream from an air purification system which wag designed
to remove SO.sub.2, such a detection of the off-set voltage could
be used to indicate a breakthrough of SO.sub.2 at the point of gas
sampling. The flow was continued until the adsorbent portion in the
pipe section was saturated with SO.sub.2 and SO.sub.2 was detected
at the exit end of the detector, at which time the off-set voltage
of the Wheatstone bridge began to decrease as the resistance of the
reference sensing element began to decrease more rapidly.
EXAMPLE 3
Detection of Ammonia
A detector having the same construction and dimension as that used
in Example 1 was tested for the detection of ammonia in a stream of
air. The sensing elements in this example were made of FM5/250 ACC
impregnated with about 15 percent (by weight) citric acid. The pipe
section contained about 100 g of a granular carbon impregnated with
15 percent (by weight) citric acid. Air at a flow rate of about 32
l/min. and about 50% RH was conducted through the detector
overnight. Then the Wheatstone bridge was balanced and ammonia was
introduced into the air stream to obtain a concentration of about
500 ppm by volume. The Wheatstone bridge immediately showed a large
negative off-set voltage, as is shown in FIG. 4, indicating a large
change in the electrical resistance of the first sensing element.
The resistance of the first sensing element immediately increased
from 18.4 ohm to 24.8 ohm while that of the second (reference)
element increased from 20.0 ohm to 25.9 ohm. If this stream of air
had been a split stream from an air purification system which was
designed to remove ammonia, such a detection of the off-set voltage
could be used to indicate a breakthrough of ammonia at the point of
gas sampling. The flow was continued until the adsorbent portion in
the pipe section was saturated with ammonia and ammonia was
detected at the exit end of the detector, at which time the off-set
voltage of the Wheatstone bridge began to increase as the
resistance of the reference sensing element began to increase more
rapidly.
EXAMPLE 4
Detection of Sucrose in an Aqueous Solution
A detector having the same construction and dimension as that used
in Example 1 was tested for the detection of sucrose in a stream of
water. The sensing elements in this example were FM5/250 ACC. The
pipe section contained about 150 g of a granular carbon. Deionized
water at a flow rate of about 90 cm.sup.3 /min. was conducted
through the detector overnight. Then the Wheatstone bridge was
balanced and a five-percent (by weight) sucrose aqueous solution
was introduced into the detector. The Wheatstone bridge immediately
showed a positive off-set voltage, as is shown in FIG. 5,
indicating a change in the electrical resistance of the first
sensing element. Data for the resistances of the sensing elements
were not recorded. However, the increase in the off-set voltage
indicated that the magnitude of the change in the resistance of the
first sensing element is larger than that of the second or
reference sensing element. If this stream of solution had been a
split stream from a water purification system which was designed to
remove sucrose, such a detection of the off-set voltage could be
used to indicate a breakthrough of sucrose at the point of
sampling. The flow was continued until the granular carbon
adsorbent portion in the pipe section was substantially saturated
with sucrose and sucrose was detected at the exit end of the
detector, at which time the off-set voltage of the Wheatstone
bridge began to decrease as the resistance of the reference sensing
element began to approach that of the first sensing element.
EXAMPLE 5
Detection of Isopropyl Alcohol ("IPA") in an Aqueous Solution
A detector having the same construction and dimension as that used
in Example 1 was tested for the detection of IPA in a stream of
water. The sensing elements in this example were FM5/250 ACC. The
pipe section contained about 150 g of a granular carbon. Deionized
water at a flow rate of about 90 cm.sup.3 /min. was conducted
through the detector overnight. Then the Wheatstone bridge was
balanced and a 9.1% (by volume) IPA aqueous solution was introduced
into the detector. The Wheatstone bridge immediately showed a
positive off-set voltage, as is shown in FIG. 6, indicating a
change in the electrical resistance of the first sensing element.
Data for the resistances of the sensing elements were not recorded.
However, the increase in the off-set voltage indicated that the
magnitude of the change in the resistance of the first sensing
element is larger than that of the second or reference sensing
element. If this stream of solution had been a split stream from a
water purification system which was designed to remove IPA, such a
detection of the off-set voltage could be used to indicate a
breakthrough of IPA at the point of sampling. The flow was
continued until the granular carbon adsorbent portion in the pipe
section was substantially saturated with IPA and IPA was detected
at the exit end of the detector, at which time the off-set voltage
of the Wheatstone bridge began to decrease as the resistance of the
reference sensing element began to approach that of the first
sensing element.
EXAMPLE 6
Detection of Butane Breakthrough from a Purification System using
Granular Activated Carbon
A granular activated carbon bed having a diameter of about 2 feet
and a depth of about 15.5 inches was used to purified a stream of
air contaminated with n-butane. The carbon bed was equipped with
sample taps at 3, 9, and 15 inches from the inlet end. A second
embodiment of the detector of the present invention was used to
detect the adsorption front of n-butane as it progresses from the
inlet to the outlet end of the carbon bed. FIG. 7 shows the
cross-sectional view of a detector cell of the second embodiment of
the present invention. Each sensing element is disposed in a
separate cell housing which comprises matching halves 100 and 102
which are made of, for example, a polymeric material. The ACC
sensing element 140 is disposed between the two cell housing halves
and secured in place by bolts 150. The ACC sensing element is
provided with a brass electrode 142 to enhance the electrical
contact between the sensing element 140 and the bolts 150, which
also serve to connect electrical leads to a resistance measuring
device. A stream of fluid mixture, the change in the composition of
which is desired to be detected, is conducted through the cell
housing past the ACC sensing element. As a minor component of the
fluid mixture adsorbs in the ACC material of the sensing element,
its resistance changes and a detection is indicated. A Wheatstone
bridge was used to detect a change in the resistance of the sensing
element at the 9-inch tap relative to the reference sensing
element.
One detector cell was provided at the 9-inch tap and another
serving as the reference was provided at the outlet end of the
granular carbon bed. Air at about 27% RH and about 27 .degree. C.
containing about 950 ppm (by volume) n-butane was conducted through
the carbon bed at about 100 ft.sup.3 /min. A split stream of 10
l/min. was drawn into each detector cell. Another split stream at
the 9-inch tap was also directed to a hydrocarbon analyzer equipped
with a flame ionization detector to measure the concentration of
n-butane. FIG. 8 shows the Wheatstone bridge off-set voltage in
relation to the n-butane concentration. When the n-butane
concentration began to increase sharply at the 9-inch tap, the
Wheatstone bridge off-set voltage also increased sharply,
indicating a substantially simultaneous correspondence between the
concentration change and the voltage change.
The foregoing examples show that the detector of the present
invention is capable of indicating changes in concentrations of a
wide variety of compounds in both gas and liquid mixtures. Thus,
detectors of the present invention can be successfully employed to
indicate breakthrough of components of fluid mixtures.
While the foregoing has described the preferred embodiments and
modes of operation of the present invention, it should be
appreciated that numerous variations, changes, and equivalents may
be made to these embodiments and modes of operation without
departing from the scope of the present invention as defined in the
following claims.
* * * * *